Telephone systems in the United States and in many other countries were originally designed solely for the transmission of analog voice signals between subscriber locations and a central office. At the central office, signals from and to the various subscriber locations are typically handled by suitable switching systems. Connection between the central office and each subscriber location is typically by means of a single subscriber loop--usually a twisted pair of insulated copper wires.
With the coming of the "information age," telephone companies have sought ways to increase the level of service to their subscribers. One way of doing this is to increase the capacity of subscriber-to-central office communication by adding subscriber loops. As one might imagine, however, the cost of installing additional subscriber loops for all subscribers in an existing telephone system is prohibitive.
Many telephone companies have, accordingly, explored the use of digital signalling techniques along with multiplexing and time compression in order to cram more information into signals travelling between subscribers and the central office. Since digital transmission techniques via microwave and other long distance signalling systems are already employed, and inasmuch as digital switching is utilized in many modernized central offices, the digitizing of the subscriber loop affords evident advantages. Nevertheless, although the prior art is replete with various approaches to the subscriber loop digitization problem, the difficulties thus far encountered have prevented practical commercial fulfillment of this objective.
By way of example, a bidirectional subscriber loop transmission system is described by Soejima, et al., "Experimental Bidirectional Subscriber Loop Transmission System," IEEE Transactions on Communications, Vol. Com-30, No. 9, September 1982. This system is based on a time compression burst mode transmission scheme (ping-pong), and provides two information channels and a signalling channel. In this, and other similar systems, however, serious problems are encountered in achieving sufficient transmission quality over the distances usually encountered in connection with subscriber service. Such problems include cross talk, echoing as a result of discontinuities and taps in the line, and the natural attenuation of high frequency signals with transmission distance.
Other difficulties arise in connection with the combining of voice information with data. The data may take the form of relatively high speed data such as that utilized by a computer terminal, or relatively low speed data such as alarms or medical or police alert functions. By way of example, U.S. Pat. No. 4,476,558, to Arnon describes a digital transmission signal system employing time compression multiplexing in which burst mode or ping-pong transmission is utilized. However, only voice information is transmitted and there is no successful combination of voice and data in this system. In another example, U.S. Pat. No. 4,332,980, transmission of both voice and data is described. However, in this system, a "voice over data" technique is utilized wherein the voice information remains in analog form and wherein the data transmission is transmitted at a different and supposedly noninterfering frequency.
Nowhere in the prior art is there a teaching of a transmission method and system capable of handling both voice and data in digitized format which can be successfully applied to typical existing telephone subscriber systems. Either the capacity for handling the voice and data information is inadequate, or the transmission quality and distance is insufficient for commercial application, or both. The problems created by typical irregularities in subscriber loops resulting from discontinuities, taps, oxidation, etc. create unacceptable echo signals. Cross talk between the various data channels is in many cases unacceptable. Transmission distance to adequately serve typical subscriber locations is frequently not achieved. Quality of voice information is often unacceptable.
An additional problem encountered in converting a conventional telephone system from analog to digital operation is that, if conventional central office equipment must be replaced, there is a massive expense and reconstruction effort required. Moreover, the transition of subscribers to the additional services which digitized telephone systems make available most probably will be gradual. Thus, there is a need for an approach to digitizing a telephone system which will permit gradual changeover from conventional analog to digital operation without causing major disruptions or requiring major conversions of existing telephone equipment and service.